Don’t Subtract Add

I am learning that problems like 34 + ? = 51 and 51 – 34 = ? have the same answer.

e.g. 34 + 17 = 51 and 51 – 34 = 17

Ashleigh has $17 and needs $25 to buy the new book she wants. How much more money does she need?

First of all let’s answer this as an addition question. 17 + ? = 25

Step 1: Let’s record what she has already.

$10

$1

$1

$1

$1

$1

$1

$1

Step 2: You can see she needs $3 more to get to twenty

Amount added $3 +

$1

$1

$1Step 3: Now we need to swap these ten $1 coins for a $10 note.

Step 4: Now we have $20 so we just need to add $5 then we will have $25.

$1 $1

$1 $1

$1

$5

So $17 + $8 = $25

Ashleigh has read $17 and needs $25 to buy the new book she wants. How much more money does she need?

We can also answer this as a subtraction question. 25 – 17 = ?

Step 1: Record the amount she needs ($25).

$10

$1

$1

$1

$1

$1

Step 2: We can’t take away $7 because we only have $5 so we must split up a $10 note to get us another ten $1 coins.

So$25 - $17 = $8and$17 + $8 = $25

$1$1$1$1$1 $1$1$1$1$1$1$10

Step 3: Now we can take away $7.

Step 4: Now subtract $10. and you are left with $8. The amount Ashleigh must save.

Try and work this one out as you go.

Andrew had $23 and spent $5. How much money does he have left.As an addition question5 + ? = 23

As a subtraction question23 – 5 = ?

5 + 5 = 10

10 + 13 = 23

5 + 13 = 18

23 – 3 = 20

20 – 2 = 18

First I added 5 to get to a tidy number.

From the tidy number I added 13 to get to 23. I find it easy to add on from a tidy number.

Now I add 5 + 13 together to get my answer.

First I took away 3 to get back to a tidy number.

I need to subtract 5. I have already taken away 3 so now I subtract the remaining 2. I find it easy to subtract from a tidy number.

Notice how both methods get the same answer.

Now try these questions on your own.

Solve these subtraction questions by first changing them into addition questions.

Need some more practise? Try Pg45 of Teacher Tools Addition and Subtraction Book Numeracy resources

1) 34 – 27 = ?

2) 56 – 48 = ?

3) 72 – 69 = ?

4) 45 – 36 = ?

5) 33 – 29 = ?

Click again when you have completed each question and the answer will be displayed.

27 + 7 = 34

48 + 8 = 56

69 + 3 = 72

36 + 9 = 45

29 + 4 = 33